There are many applications for fumed metal oxides, particularly for fumed silica. Such applications include fillers for polymers such as rubber, coatings such as for paper (i.e., recording media), cosmetics, the manufacture of optical fibers and quartz glassware, thermal insulation, and chemical-mechanical polishing compositions intended for use in semiconductor manufacturing.
Fumed silica is generally produced by the vapor phase hydrolysis of chlorosilanes, such as silicon tetrachloride, in a hydrogen oxygen flame. Such processes are generally referred to as pyrogenic processes. The overall reaction is:SiCl4+2H2+O2→SiO2+4HCl
Organosilanes also have been used in pyrogenic processes for the production of fumed silica. In the vapor phase hydrolysis of organosilanes, the carbon-bearing fragments undergo oxidation to form carbon dioxide as a by-product along with hydrochloric acid.
In this process, submicron-sized molten spheres of silica are formed. These particles collide and fuse to form three-dimensional, branched, chain-like aggregates, of approximately 0.1-0.5 μm in length. Cooling takes place very quickly, limiting the particle growth and ensuring that the fumed silica is amorphous. These aggregates in turn form agglomerates of 0.5-44 μm (about 325 US mesh). Fumed silica can have high purity, with total impurities in many cases below 100 ppm (parts per million). This high purity makes fumed silica dispersions particularly advantageous for many applications.
Numerous methods have been developed in the art to produce fumed silica via pyrogenic processes. U.S. Pat. No. 2,990,249 describes a process for the pyrogenic production of fumed silica. In accordance with this process, a gaseous feedstock comprising a fuel, such as methane or hydrogen, oxygen, and a volatile silicon compound, such as silicon tetrachloride, wherein the oxygen is present in a stoichiometric or hyperstoichiometric proportion, is fed into a burner supporting a short flame having a ratio of height to diameter of about 2:1 or below. Water formed by the combustion of the fuel in oxygen reacts with the silicon tetrachloride to produce silicon dioxide particles, which coalesce and aggregate to form fumed silica. The effluent from the burner is cooled, and the fumed silica is then collected.
U.S. Pat. No. 4,108,964 describes a process for the pyrogenic production of fumed silica using organosilanes as the silicon-containing component. In accordance with this process, an organosilane, such as methyltrichlorosilane, is volatilized at a temperature above the boiling point of the organosilane. The vaporized organosilane is mixed with a gaseous fuel, such as hydrogen or methane, and an oxygen-containing gas containing from 15-100% oxygen, to form a feedstock. The feedstock is fed to a flame supported by a burner at various flow rates to produce fumed silica. The volume ratios of the individual gas components are reported not to be of critical importance. The molar ratio of the organosilane to the water-forming gases generally is said to be in the range of from 1:0 to 1:12.
Conventional processes produce fumed silica with particular, relatively narrow, ranges of aggregate size and surface area. Conventional processes do not achieve a wider range of aggregate sizes because the flame temperature largely determines the size and the structure of the aggregates produced by such processes. Specifically, when silica particles are formed in the flame, the particles are initially in a semi-liquid form and grow into aggregates up to a certain extent, depending on the flame temperature. However, due to radiative cooling or forced cooling, the temperature of the aggregates rapidly drops to the point at which the particles can no longer grow into larger aggregates, thereby limiting the aggregates to a relatively narrow size range. A process capable of producing fumed silica with a wider range of possible aggregate sizes for a given surface area would be useful, especially to produce fumed silica having a relatively larger aggregate size, which can provide many benefits, including improved rheological properties (such as higher viscosity and greater dispersibility in liquid media).
Fumed silica aggregates also have an inherent structure, because they are fractal-like, or branched, particles. Because a fumed silica aggregate is branched, and is not a solid, convex shape, it encompasses a volume associated with its apparent aggregate size, as is well-known in the field of fumed particles. This encompassed volume can be described in a ratio known as the coefficient of structure, i.e., the ratio of the aggregate's encompassed volume to its actual volume of solid silica. In a conventional fumed silica process, the coefficient of structure is set by surface area, aggregate size, and fractal dimension.
There is a need for fumed silica having different combinations of physical features, such as aggregate size, surface area, and coefficient of structure. For example, a process that raises aggregate size and coefficient of structure at the same time would enhance the improvements in rheological and reinforcement properties that would follow an aggregate size increase. Moreover, a process capable of changing the coefficient of structure independently of aggregate size could be useful in producing fumed silica with increased dispersibility, and could offer performance tradeoffs different from currently commercially available materials.